In the manufacture of crankshafts, the crank pins of the crankshaft must be ground to size on a grinding machine. To ensure that the grinding operation is ended as soon as a desired size is reached, during the machining operation it is necessary to continuously check the crank pin, in particular with regard to its diameter and circularity, within the scope of an in-process measurement method. EP-A-0859689 discloses a measuring device of this type.
From EP-A-1370391 a measuring device of this type is known which is used for in-process measurement of crank pins during a grinding operation on a grinding machine. The known measuring device has a measuring head which is movable relative to a base body of the device between a neutral position and a measuring position in which the measuring head is in measuring contact with the test piece. In particular, in the known measuring device the measuring head is connected via a linkage to a base body of the measuring device so as to be pivotable about a first swivel axis. The known measuring device also has a means for swiveling the measuring head in or out of the measuring position. For carrying out an in-process measurement at a crank pin, the measuring head is swiveled by the means provided for this purpose into a measuring position in which the measuring head, for example using a measuring prism, comes to rest against the crank pin to be measured. During the grinding operation the crank pin undergoes an orbital rotation about the rotational axis of the crankshaft. The grinding wheel remains in contact with the crank pin, and for this purpose is movably supported radially with respect to the rotational axis of the crankshaft. To ensure that measurements may be carried out at the crank pin during the entire grinding operation, the measuring head follows the motions of the crank pin. For this purpose the base body of the measuring device is connected to a base body of the grinding machine, so that during the grinding operation the measuring device is moved in the radial direction of the crankshaft, synchronously with the grinding wheel of the grinding machine.
From EP 1 063 052 B1 a measuring device is known, in which a rotary encoder is provided at a connecting point at which the measuring head is connected to the base body of the measuring device via a linkage.
An object of the invention is to provide a measuring device for in-process measurement of test pieces during a machining operation on a machine tool, in which the risk of measurement inaccuracy is reduced.
This object is achieved by the measuring device according to the invention, including a measuring device for in-process measurement of test pieces during a machining operation on a machine tool, including a base body and a measuring head which is movable relative to the base body of the measuring device between a neutral position and a measuring position in which the measuring head is in measuring contact with the test piece. There is likewise an angle sensor which operates in a machine reference-free manner provided and associated with the measuring head for detecting an angular position of the measuring head relative to the test piece, during a measuring operation.
The measuring device according to the invention likewise includes a measuring device for in-process measurement of test pieces during a machining operation on a machine tool, including a base body and a measuring head which is movable relative to the base body of the measuring device between a neutral position and a measuring position in which the measuring head is in measuring contact with the test piece. There is likewise an angle sensor which operates in a machine reference-free manner provided and associated with the measuring head for detecting an angular position of the measuring head relative to the test piece, during a measuring operation, and the angle sensor detects changes in the angular position of the measuring head relative to the test piece.
The invention is based on the assumption that generally during the measuring operation the angular position of the measuring head relative to the test piece changes, for example a test piece in the form of a crank pin of a crankshaft, and the change in the angular position being an oscillating motion which corresponds to the kinematics of a linkage, for example, via which the measuring head is connected to the base body of the measuring device.
The only exceptions thereto are measuring devices in which the linkage is configured in such a way that during the measuring operation the angular position of the measuring head does not change relative to the test piece. In this case, while the test piece is rotated about its rotational axis, a measuring probe of the measuring head associates measured values, recorded at equal time intervals, with locations of the test piece which are equidistant in the circumferential direction.
On the other hand, if the angular position of the measuring head relative to the test piece changes, which is generally the case, measured values recorded at equal time intervals in the circumferential direction, for example, by the measuring probe do not correspond to circumferential locations of the test piece which are equidistant in the circumferential direction, and instead are shifted in the circumferential direction, corresponding to the oscillating angular position of the measuring head.
To determine the contour of the test piece, for example, the measured values must be corrected so that the corrected measured values correspond to equidistant circumferential locations of the test piece.
The invention is based on the concept of avoiding measurement errors which result from; i.e., in connection with, this correction.
In the device known from the prior art, the rotational position of a crankshaft is detected, and the correction of the angular position is determined by computer, using the kinematics of the measuring head specified by mechanical factors. This correction is based on the assumption that an angular position of the measuring head is uniquely associated with every rotational position of the crankshaft.
The invention departs from the concept of carrying out the correction by computer. Rather, it is based on the concept of detecting changes in the angular position of the measuring head relative to the test piece using an angle sensor which is adapted thereto and associated with the measuring head. Accordingly, the invention provides that an angle sensor is associated with the measuring head for detecting the angular position of the measuring head, in particular changes in the angular position of the measuring head relative to the test piece, during a measuring operation.
In this manner, in particular changes in the angular position of the measuring head are detected by sensor. The changes in angular position detected by sensor may be included with high accuracy into the correction of the measured values or their association with circumferential locations of the test piece.
The measuring device according to the invention has the advantage in particular that changes in the kinematics of the measuring head, which are caused, for example, by wear of a linkage which connects the measuring head to the base body of the measuring device, or also which occur when the measuring device is modified for adjustment to a crankshaft having different dimensions, do not adversely affect correction of the association with the measured values. The accuracy of the correction of the association with the measured values may thus be increased.
Another advantage of the invention is that it may be used without modification when a component of a grinding machine to which the measuring device is attached moves during the measuring operation. Corresponding motions of the component bearing the measuring device, and thus motions of the measuring device, may be planned and desirable. However, the motions may also be random and undesirable. Regardless of the cause of the motions in question, they do not influence the measuring accuracy of the measuring device according to the invention since, thanks to the use of a dedicated sensor which is associated with the measuring head, changes in the angular position are detected regardless of their cause. The flexibility of the measuring device according to the invention is thus increased with regard to their use in cooperation with a machine tool.
According to the invention, it has been achieved that the measured values recorded by a measuring probe of the measuring head are evaluated by first compensating for or correcting the change in angular position of the measuring head relative to the test piece during the measuring operation. The compensated or corrected measured values may then be further processed for reconstructing the correction of the test piece, for example.
In particular, the angle sensor, which according to the invention is associated with the measuring head, is an angle sensor which operates in a machine reference-free manner. According to the invention, an angle sensor which operates in a machine reference-free manner is understood to mean an angle sensor which detects the angular position or changes in the angular position of the measuring head independently of a reference which is defined by the measuring device or the machine tool. Accordingly, sensors whose functional principle is based on interaction with a magnetic field or the earth's gravitational field angle, for example, are regarded as sensors which operate in a machine reference-free manner. In contrast, an angle sensor which does not operate in a machine reference-free manner is understood to mean a sensor which is known from the prior art and which is used to sense rotations of the test piece relative to the grinding machine so that, based on this information and known kinematics of the measuring head during the measuring operation, changes in the angular position of the measuring head are computed.
In principle, according to the invention it is sufficient for a single angle sensor to be associated with the measuring head. If necessary or desired, depending on the particular requirements, at least two angle sensors may also be associated with the measuring head.
Depending on the particular requirements and circumstances, the sensor principle; i.e., the principle underlying the sensor, of the angle sensor provided according to the invention may be selected within a broad range, as long as it is ensured that the angular position or changes in the angular position of the measuring head relative to the test piece during the measuring operation is/are determinable with an accuracy that is appropriate for the application.
In this regard, one advantageous further embodiment of the invention provides that the angle sensor is an inclination sensor. A corresponding inclination sensor is essentially an angle measuring device with reference to the surface of the earth, and measures the deviation from the horizontal or vertical. The reference is the earth's gravitational field, so that a corresponding sensor operates in a machine reference-free manner.
One extremely advantageous further embodiment of the invention provides that the angle sensor is an acceleration sensor. An acceleration sensor measures the acceleration, for example, by determining the inertial force acting on a test mass. According to the invention, the static or dynamic acceleration may be used for this purpose. One particular advantage of this embodiment is that corresponding acceleration sensors, which are also referred to as accelerometers, acceleration meters, and G sensors, are available as relatively small, versatile, and inexpensive standard components. Instead of acceleration sensors, according to the invention other inertial sensors may be used.
Another advantageous further embodiment of the invention provides that the angle sensor has a fiber gyro, also referred to as a fiber optical gyro or interferometer fiber optic gyroscope (IFOG). The operating principle of a fiber gyro is based on the interference of two light beams circulating in opposite directions in a wound glass fiber. In terms of function, such a fiber gyro is based on the principle of the Sagnac interferometer. Corresponding fiber gyros are likewise available as relatively inexpensive components. Since they do not operate in a completely drift-free manner during operation, calibration is necessary to ensure that the measuring results recorded during a measuring operation are not distorted by drift effects. Instead of a fiber gyro, a laser gyro may be used, the function of which is likewise based on the Sagnac effect. The mode of operation of a fiber gyro or laser gyro in particular is generally known to one of ordinary skill in the art, and therefore is not explained in greater detail herein.
Another embodiment which differs with regard to the operating principle of the sensor provides that the angle sensor is a magnetic field sensor. The operating principle of such a magnetic field sensor may, for example, be based on detection of the position of a permanent magnet which moves in a magnetic field generated by a coil. In other respects, the design of such magnetic field sensors is generally known to one of ordinary skill in the art, and therefore is not explained in greater detail herein.
In addition, further sensor principles may be used according to the invention. For example, an optical sensor system may be used, by means of which the angular position or changes in the angular position of the measuring head are optically detected. In this regard it is possible, for example, to detect changes in the angular position of the measuring head using an electronic camera, and to determine the changes in angular position from the output signal of the camera, using image processing algorithms. The configuration of the angle sensor or the angle sensors may be selected within a broad range, depending on the particular requirements. In this regard, one advantageous embodiment provides that the angle sensor is situated on the measuring device.
One extremely advantageous further embodiment of the invention provides that the angle sensor is situated on the measuring head or on a part of a linkage that is rigidly or practically rigidly connected to the measuring head, by means of which the measuring head is connected to the base body of the measuring device. In this embodiment, the angle sensor is directly connected to the measuring head or to a part of the linkage rigidly connected thereto. The greatest possible accuracy is thus achieved in detecting changes in the angular position of the measuring head, which is not affected by wear, for example of articulated connections of a linkage which connects the measuring head to the base body of the measuring device.
Another advantageous further embodiment of the invention provides that the angle sensor is connected to an evaluation device which determines the angular position or changes in the angular position of the measuring head relative to the test piece as a function of at least one output signal of the angle sensor.
Another advantageous further embodiment of the invention provides that a sensor which determines the particular rotational position of a test piece which rotates about a rotational axis during the measuring operation is connected to the evaluation device. If the test piece is a crankshaft, for example, which rotates about a rotational axis during a grinding operation, the particular instantaneous rotational position of the crankshaft may be sensed using an appropriate rotational position sensor. A corresponding signal may then be transmitted to the evaluation device, which, as a function of these output signals and the output signals of the angle sensor, establishes a unique association of measured values, which are recorded using a measuring probe of the measuring head, with corresponding circumferential locations of the crankshaft.
The invention is explained in greater detail below with reference to the accompanying highly schematic drawings which illustrate an embodiment of a measuring device according to the invention. In this regard, all features that are described, illustrated in the drawings, and claimed in the claims constitute the subject matter of the invention, taken alone or in any given combination, independently of their combination in the claims and their dependencies, and independently of their description or illustration in the drawings.
Relative terms such as left, right, up, and down are for convenience only and are not intended to be limiting.